THE DEFINITIVE TRUCKING SITE

Nov/Dec 2007

transmissions

A cut-away view of a torque convertor – this is where torque (Nm) gets instantly multiplied – and there’s no slip or loss on a gradient from a standing start. It’s a pricey component but ideal for continuous stop-and-launch situations

It’s not often that one gets the opportunity to drive a range of 20 truck and bus applications equipped with a variety of transmissions. A special fleet owner and press event in Hungary at the Euro-ring Test Track near Budapest proved to be a well organised event where Allison Transmission guests could drive this 20- truck fleet in diverse operating conditions and get first-hand experience. Is there really a difference between an automated manual-shift transmission (AMT) and a fullautomatic torque convertor transmission? What are the acceleration characteristics comparing AMT to full auto, and what is ‘clutch fudge’? FleetWatch technical correspondent Dave Scott was there to answer these and other question.

Refuse compactors are regarded as an ideal Allison Transmission application. It’s one of the harshest stop/start applications to punish a whole driveline

TO TAKE JUST one step back into history, today there are no trucks on the road equipped with a ‘crash-box’. ‘Crashbox’ is exactly what it means – gear ratios are changed by sliding different gears in and out of mesh with each other creating a noisy ‘crashing’ effect.

This very old technology – stuff from the 1st half of the 20th Century – gave way to constant mesh designs that are even used today in auto-shift transmissions without synchromesh cones. All manual gearboxes are constant mesh in design – those without synchronizing rings have been misnamed as ‘crashboxes’ simply because gearshifts are an art without synchro-cones to assist a sliding sleeve engage a gear on the output shaft. 
 

Allison Transmissions are widely used in off-road applications, especially in military units.

Here is a typical example of a truck rear end that needs driving skill and protection. Reversing this into loading bays requires slow manoeuvring that only a torque convertor can provide. For a manual or automated shift transmission this can be very harsh on clutches – in the case of a manual transmission one slip of a clutch in reversing can damage truck rear assemblies. Just look at the evidence on our roads.

MT, AMT or full auto
Manual transmissions (MT) dominate the trucking scene but automated manual transmissions (AMT) are gaining ground, still using a clutch while gear-changes are effected automatically via electronically controlled, air-assisted ‘mechatronics’. 

The torque-convertor, fully automatic transmission, pioneered in trucks and buses by Allison Transmission, has been around for many years well before the arrival of AMT. Allison’s first fully-automatic, production-series transmission was manufactured in 1948 for coaches and off-highway vehicles – trucking applications only arrived in the market from 1956 onwards. 

A constant mesh transmission without synchro-cones to assist gear changing is still around on South Africa’s roads but in dwindling numbers because without synchromesh, using a standard Eaton transmission, gear-shifts are a skilled driving art. As the SA skilled driver pool diminishes, this type of gearbox design becomes more of a hazard than a reliable, cost-effective, driveline tool. 

AMT has saved the day for Eaton. Ever since Freightliner adopted the automated manual transmission – SmartShift in terms of their branding – the AMT box has taken off in the over 56t gross combination mass (GCM) segment and in fact, has probably rescued Eaton from extinction on our roads, enhancing the two USA brands – Freightliner and International.
 

Truck acceleration tests proved that a torque convertor automatic can out-accelerate an AMT unit very time – even where the AMT unit is powered by an engine with far more kW and Nm

But the word ‘automatic’ does not imply that an AMT unit has identical performance characteristics in specific applications to the torque convertor auto-box. When it comes to very frequent vehicle launch and stop operations, extreme off-road conditions, or constant traffic gridlock situations in distribution tasks, then a torque convertor is superior to a an electronicallyoperated clutch. 

What’s driving Allison Transmission sales in South Africa? It’s a combination of application requirements aggravated by lack of driver skill together with as many as three drivers using one truck in a 24- hour shift. At dairy produce distribution companies such as Clover, where there are over 100 Allison fully automatic gearboxes in operation, they actually blank off the 1 and 2 gear positions on the automatic shift lever to reduce a driver’s option to forward (D), neutral (N) or reverse (R). As Dup du Plessis, Technical Manager of Fast-’n-Fresh puts it: "The objective is to minimise the options associated with and deskill the tasks of truck driving." 

An Allison auto-box really proves its torque convertor benefits when reversing into difficult loading bays, where there may even be a gradient to overcome. Driving both AMT and full automatic transmission units recently at the Euro-Ring test track gave ample proof of the superiority of the torque convertor automatic over the AMT unit in reversing situations. Instead of constantly engaging or disengaging a clutch as with an AMT unit, the Allison torque convertor takes up the slip while still multiplying torque to allow for a smooth flow of engine Nm into the driveline as a driver inches backwards. An auto-box with torque convertor simply provides more control over reversing and is a key factor in minimising rear end damage from badly managed reverse shunts.
 

SA visitors to the Allison Transmission event in Hungary put on a show of team spirit

Full auto – off-road favourite
OFF-ROAD EARTHMOVING  vehicle manufacturers such as Bell articulated dump trucks and Western Star also install full auto Allison transmission for the rigours of stop-start, severe gradient applications where the advantage of a full-torque power-take-off also comes into play. According to Duncan Prince, Western Star product manager: "An Allison auto-box is ideal where terrain or load conditions are such that pullaway and gearshifts are strenuous. This can be in a number of areas. If a vehicle has to operate in soft sand or thick mud, launching from standstill is very tough on clutches and gearshifts are often difficult because breaking torque to the wheels for a gearshift stops the vehicle. This situation is also emulated when pulling heavy loads on steep inclines. A vehicle is ‘gear-bound’ because it can't shift gear without the load stopping the vehicle dead. 

"An Allison auto, on the other hand, never interrupts the flow of torque to the wheels while it shifts gear, the advantage of this being that a vehicle that is equipped with an Allison will up-shift under load or incline or soft ground conditions where a clutchequipped vehicle would never make the shift. An Allison auto box truck will run faster and use less fuel in these conditions because it is running in a higher gear." 

And then – to coin an expression – ‘clutch fudge’ compounds the problem of vehicle start-ability on a gradient 

Press and fleet owner transfers from Budapest to the Allison Transmission Zentgotthard Plant near the Austrian border were done on a Russian Mi8 helicopter.....eisshhhhh!

Allison are concentrating on the bus-train niche with a hybrid  transmission application that has proved successful

What is ‘clutch-fudge’?
A gradient describes the inclination of a road to the horizontal plane. The value is expressed in % – for example, a gradient of 100% is equivalent to an angle of 450. The accompanying Chart A expresses the concept. 

1ST GEAR GRADE-ABILITY on a specification sheet means the gradient that a truck can handle on the move at maximum GVM at full torque – without de-rated loss for altitude – and without taking into account any road surface rolling resistance. A road surface of loose sand equals a rolling resistance of up to 10% that must be added to the grade resistance to determine the total gradeability required. But this still does not include a factor for start-ability should the truck stall on the gradient and need to launch itself from a standing start. 

It’s an often-overlooked fact that vehicle % grade-ability does not allow for % gradient start-ability. This is where a standard clutch slips on launch from stand-still and reduces 1st gear grade-ability by anywhere up to 12%. A standard clutch is not a torque convertor – it’s there to interrupt Nm flowing into a gearbox and unlike an automatic transmission with torque convertor, a clutch does not multiply torque when a truck starts against a gradient. 

This is ‘clutch-fudge’ where gradeability spec comparisons between trucks equipped with automatic torque convertors and manual transmissions overlook the startability factor. If the truck stalled in Chart A on a 30% gradient, it would require a 1st gear grade-ability of at least 40% to get moving again and not 30% as indicated from the chart. A poor road surface compounds the  problem even further. 

It’s no wonder that overlooking the ‘clutch-fudge’ factor has burnt out many clutches on what would be perceived to be average and not severe gradients. Start-ability is another reason why automatic torque convertor transmissions are the favourite for severe off-road conditions. But even around the suburbs of the Durban/Pinetown region, there are some very ‘interesting’, hard-surface gradients for trucks to negotiate where startability can be easily ‘fudged’. 

There are a number of transmissions equipped with a crawler gear – a deep 1st gear ratio that is designed to deal with startability emergencies and move a truck out of a ‘hole’ but it is for that only and is not designed to be shifted on the move into the next gear. Use the crawler gear to get to a less severe gradient and then start off in 2nd. This is where an Allison auto box has an advantage as it can effect a shift change at very slow speeds.
 

In this example height (H) is 30m while the horizontal plane (L) is 100m so the required truck grade-ability is expressed as: H x 100 ÷ L = 30 x 100 ÷ 100 = 30%

Is the ‘fudge’ factor taken into account? 
When writing up tender documents, has the ‘clutch fudge’ issue been properly addressed? Do truck sales people really ask about gradient startability when conducting vehicle selection processes for a client? When assessing abnormal clutch failures, has the ‘fudge’ factor even been discussed? I think not.

Remember that clutch failures are more often caused by reversing than by going forward. This has been regarded as a major factor in favour of the Allison automatic in some fleets where rear-end loading-bay shunting damage has been minimised because of the ease of reversing with a torque convertor. 

In the long term, AMT units will take over from standard synchromesh transmissions in long-haul applications and even in distribution vehicles but will have their limitations in comparison to a torque convertor full auto transmission for certain strenuous applications. An Allison automatic will also out-accelerate an AMT unit even when driven by a more powerful engine – so if productivity factors are reduced to seconds in a constant stop-andlaunch operation - such as a refuse compactor - the torque convertor automatic can produce more payload units in a day. 

Manufacturers of automated manual shift transmissions cannot claim to be all things to all applications when multiplying driveline torque in trucking. There’s a distinct difference between an AMT and a torque convertor auto box – I have had the privilege of being able to go beyond paper and drive these units for comparison purposes.
  

"Allison transmission automatic torque convertor units are ideal for military and paramilitary applications," says Pieter Grundlingh – Manager: Materials & Procurement at BAE Systems. "There can be no compromise about acceleration and driveline reliability in extreme operating conditions."